The successful implementation of a lightwave communication system requires the manufacture of high quality lightguide fibers having mechanical properties sufficient to withstand stresses to which they are subjected. Each fiber which is made of glass must be capable over its entire length of withstanding stresses that it will encounter during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundred transmission circuits. The failure of lightguide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength from that of the pristine unflawed glass.
The potential strength of a lightguide fiber is realized only if it is protected with a relatively thin layer of a suitable coating material soon after it has been formed, such as by drawing it from a preform. This coating which has a thickness of about 0.005 cm serves to prevent airborne particles from impinging upon and adhering to the surface of the fiber which would serve to weaken it. Also, the coating shields the fiber from surface abrasion, which could be inflicted by subsequent manufacturing processes and handling during installation, provides protection from corrosive environments, and spaces the fibers in cable structures.
Important properties relating to the coating are its concentricity, and its thickness. An off-centered fiber in the coating may not adequately protect the fiber surface which could have an adverse effect on fiber strength and microbending loss. The coating must be thick enough to adequately cover and protect the surface of the fiber, but not so thick that it impairs subsequent manufacturing operations and/or connectorization.
In one process, the coating is applied by advancing the lightguide fiber through a reservoir of an open cup applicator containing a liquid polymeric material. Typically, the fiber enters the coating material through a free surface, and exits through a relatively small die orifice at the bottom of the reservoir.
Uniform wetting of the fiber during the coating process is largely affected by the behavior of an entrance meniscus which exists where the fiber is advanced through the free surface of the coating material in the reservoir. As is well known, the wetting characteristics of two materials such as a liquid coating material and glass, depend on surface tension and chemical bonds which are developed between the two materials.
The wetting characteristics are affected by a pumping of air into the meniscus. The fiber pulls a considerable amount of air into the coating material as it enters the free surface of the coating applicator. The entrance meniscus is drawn down with the moving fiber, instead of rising as it does under static conditions.
As the line speed is increased, the meniscus extends downwardly and develops into a long, thin column of air which surrounds the fiber and which is confined by surface tension in the coating material. The meniscus becomes unstable, oscillating between a fully developed state with circulation and a relatively small size with little or no circulation.
Should the column of air be caused to extend completely through the coating material to the die orifice, the meniscus collapses and the fiber no longer contacts the coating material. A meniscus may be reformed and the process of collapse repeated. If meniscus collapse has occured, the fiber may be coated but there is insufficient wetting to obtain a uniform covering of substantially the entire outer surface of the fiber. As a result, the strength and size of the fiber are adversely affected and the likelihood for damage and transmission losses is increased.
Meniscus collapse may be caused as a result of higher line speeds and the relatively high temperature of the fiber as it emerges from a drawing furnace. As the fiber is moved from the furnace into a reservoir of coating material, the air surrounding the fiber expands and prevents contact of the coating material with portions of the fiber. If the fiber is cooled sufficiently prior to its passage through the coating reservoir, meniscus collapse is avoided.
The prior art has addressed this problem. A solution is to space the coating applicator a sufficient distance from the drawing furnace so that the fiber is cooled by ambient air. Then when the fiber enters the coating applicator, it is sufficiently cool and meniscus collapse is avoided. In some instances however, it may not be feasible to increase the distance between the furnace and the coating applicator. Physical restrictions of the building in which fiber drawing apparatus is located may preclude the use of such a solution. Possibly, sufficient cooling time could be realized with the present arrangement by reducing line speed. Because meniscus collapse is affected by line speed as well as by the temperature of the fiber, a reduction in line speed is particularly helpful in decreasing the probability of meniscus collapse. However, from an economic standpoint, a decrease in line speed is not a desirable solution.
What is needed and what is not provided by the prior art is a coating arrangement in which the drawn fiber is provided with a coating having sufficient thickness without requiring undue space between the drawing furnace and the coating cap.